High biodiversity and potent anti-listerial action of complex red smear cheese microbial ripening consortia

Abstract

When Listeria monocytogenes EGDe (serovar 1/2a) was cultivated in cell-free supernatants prepared from red smear cheese microbial ripening consortia grown for 8 h in liquid medium, 8 out of 49 supernatants exhibited a bactericidal activity, sometimes even reducing the inoculum of L. monocytogenes from 5 × 107 CFU/ml to zero after 24 h of incubation. Another five consortia displayed a bacteriostatic capacity. No inhibition in supernatants was observed when the complex consortia were incubated for a 10-min period only, indicating that the activity depends on actively growing consortia. Consortia displayed a very high biodiversity (Simpson’s strain diversity index up to 0.97, species diversity up to 0.89). However, biodiversity did not correlate with anti-listerial activity. There was no obvious similarity between the anti-listerial consortia studied, and no general difference in comparison to non-inhibitory communities. The proportion of lactic acid bacteria (LAB) in the consortia ranged between 3 and 45%. Therefore, the presence of 23 different LAB bacteriocin genes was investigated using specific PCR primers, identifying one to five bacteriocin genes in several consortia. In situ transcription of lactococcin G mRNA on the cheese surface was demonstrated by RT-PCR in five samples, but this bacteriocin displayed no anti-listerial activity. Supernatants subjected to thermal and enzymatic treatment suggested the presence of heat-stable, non-proteinaceous molecules as well as heat-labile compounds which are sensitive to proteolytic digestion. Probably, substances other than LAB bacteriocins are responsible for the pronounced antilisterial action of some supernatants.